1. Field
The invention is in the field of processes for refining silver, particularly for removing base metal impurities from silver.
2. State of the Art
The American Society for Testing and Materials (ASTM) has established a standard for commercial grade silver. This standard, referred to as Specification B413-69, establishes maximum amounts of certain impurities which may be present. For example, the ASTM Specification B413-69 for Grade 99.90 silver provides that the sample must contain at least 99.90 percent silver, but must contain no more than 10 parts per million (ppm) bismuth, 800 ppm copper, 20 ppm iron, and 250 ppm lead.
One method of refining crude metallic silver to commercial purity is by electrolysis. Electrolytic refining is accomplished by casting the crude silver into anodes and applying current, whereby the silver from the anode precipitates as silver crystals. The crystals are then melted and cast into bars. This method is effective to remove impurities from the silver, but has the disadvantages of being slow and requiring a relatively large amount of crude silver for effective operation.
A pyrometallurgical refining method is a less expensive, and thus preferred, method of refining crude silver to commercial purity. This method requires melting the crude silver, treating it with a gaseous chemical to remove impurities, and then pouring the purified sliver into bar molds. This method is faster than electrolytic refining and requires less of the crude silver. As a result, the valuable silver is held up in inventory for a considerably shorter period of time than in an electrolytic refining operation, and other significant cost savings are also realized.
However, existing pyrometallurgical refining techniques prior to the present invention were not capable of removing bismuth to a commercially acceptable degree. Thus, where crude silver has contained significant amounts of bismuth, it has heretofore been necessary to employ the more costly electrolytic refining process.